[Show abstract][Hide abstract] ABSTRACT: Kinetics of cellulose hydrothermal carbonization was investigated over different reactions times and temperatures, developing a first order-reaction model, where chemical and heat transfer processes were connected and resolved simultaneously. According to the model, mass species evolutions are described by sigmoid curves and the presence of an induction period confirms the importance of taking into account the heat-up time and not only the T-constant period during the experiments. Moreover, the model indicates that temperature plays a main role on cellulose HTC (hydrothermal carbonization) reaction, affecting both pre-exponential and exponential factors of the kinetic constant. Finally, it is interesting to remark, from a practical viewpoint, that the model was able to describe the changes in H/C, O/C and high heating value taking place during cellulose hydrocarbonization, allowing the chemical composition and energy densification to be forecasted.
[Show abstract][Hide abstract] ABSTRACT: Abstract Activated carbons saturated with p-nitrophenol were regenerated under subcritical conditions by means of wet oxidation, analyzing the effect of temperature (160-200 °C) and oxygen partial pressure (3-12 bar). The research intended to achieve two main objectives: (a) to restore the carbons adsorption performance and (b) to get insight about the p-nitrophenol oxidation process and how it can be influenced by the presence of the adsorbent. Based on the contaminant oxidation mechanisms, which was defined as a series of radical processes, a kinetic model was developed. The model was created by COMSOL MULTIPHYSICS software, and allowed obtaining characteristical parameters. Further comparison between theoretical and experimental results showed the suitability of the model in the whole range of experimental domain. Subsequent application of this model to regeneration experiments confirmed the need of developing a new model in the latter case, since the presence of carbon involved a significant modification of kinetic parameters, which was related to the catalyzing action of the adsorbent. It was also found that both temperature and pressure affect the p-nitrophenol degradation process as well as porosity regain of the carbons. Values of p-nitrophenol regeneration efficiency up to 87% for the highest values of temperature and oxygen partial pressure.
Full-text · Article · May 2015 · Journal of Supercritical Fluids The
[Show abstract][Hide abstract] ABSTRACT: This work aims to investigate how the carbon densification provided by hydrocarbonization (HTC) of biomass can improve its subsequent exploitation by steam gasification processes, in comparison with traditional pyrolysis. For this task, olive waste was subjected to HTC and pyrolysis. Then the hydrochar (HC) and char (PYR), as well as the pristine precursor (BIO), were steam gasified in a lab-scale experimental installation. The influence of steam flow rate (0.5–1 g min−1) and temperature (700–900 °C) on the composition of the gas evolved (H2, CO, CO2, CH4) was investigated.
Full-text · Article · Mar 2015 · Journal of Analytical and Applied Pyrolysis
[Show abstract][Hide abstract] ABSTRACT: The influence of the variables biomass/water ratio (1.1-12.3%), temperature (150 - 250 °C) and residence time (3.2 - 36.8 h) on the hydrothermal carbonization of olive stone has been studied. The implementation of a Design of Experiments-Response Surface Methodology approach allowed the process to be optimized in terms of reactivity (solid yield) and energy densification (increase in higher heating value), and the importance of each variable in the process to be identified. Solid yield ranged between 30.95% and 55.75% and HHV from 22.2 to 29.59 MJ kg−1. Interactions between the different variables involved were investigated to provide more precise control of the overall process so as to satisfy any given energy densification requirement of the final hydrochar.
No preview · Article · Feb 2015 · Journal of Analytical and Applied Pyrolysis
[Show abstract][Hide abstract] ABSTRACT: Biomass plays an important role as an energy source, being an interesting alternative to fossil fuels due to its environment-friendly and sustainable characteristics. However, due to the exposure of customers to emissions during biomass heating, evolved pollutants should be taken into account and controlled. Changing raw materials or mixing them with another less pollutant biomass could be a suitable step to reduce pollution. This work studied the thermal behaviour of olive pomace, pyrenean oak and their blends under combustion using thermogravimetric analysis. It was possible to monitor the emissions released during the process by coupling mass spectrometry analysis. The experiments were carried out under non-isothermal conditions at the temperature range 25-750 °C and a heating rate of 20 °C·min-1. The following species were analysed: aromatic compounds (benzene and toluene), sulphur emissions (sulphur dioxide), 1,4-dioxin, hydrochloric acid, carbon dioxide and nitrogen oxides. The results indicated that pollutants were mainly evolved in two different stages, which are related to the thermal degradation steps. Thus, depending on the pollutant and raw material composition, different emission profiles were observed. Furthermore, intensity of the emission profiles was related, in some cases, to the composition of the precursor.
Full-text · Article · Oct 2014 · International Journal of Molecular Sciences
[Show abstract][Hide abstract] ABSTRACT: In this work, the cyclic thermal regeneration of activated carbons saturated with p-Nitrophenol was analyzed with two complementary views: the examination of the process itself and the porosity modifications found in the adsorbents.
The process, which was followed by TG/DTG/TDA analysis, showed that the regeneration process is highly dependent on the number of uses of the carbon. As it is reused, it becomes more and more resistant towards the heat treatment.
Regarding the porosity, the fresh adsorbent (Vmi = 0.42 cm3 g−1), underwent a significant porosity decrease after saturation (Vmi = 0.04 cm3 g−1), mainly due to micropore blockage. The subsequent regeneration succeded to render around 70% of the adsorbent N2 uptake. However, further uses of the carbon resulted in a progressive decay of the adsorbent capacity, with a marked porosity widenning. These features were further investigated by means of Scanning Electron Microscopy and Infrared Spectroscopy.
No preview · Article · Mar 2014 · Journal of Analytical and Applied Pyrolysis
[Show abstract][Hide abstract] ABSTRACT: This work deals with the regeneration of p-Nitrophenol (PNP) loaded Activated Carbons by means of aqueous thermal desorption.Continuous desorption experiments were performed under different temperature (140–200 °C) and stirring (100–600 rpm) conditions. The study was made with a double point of view; firstly, desorption kinetics was analyzed by applying non-linear Lagergren model. Secondly, the improvements on the textural parameters of the regenerated adsorbents as well as the regeneration efficiency were evaluated.The results obtained revealed that the final amount of PNP desorbed was temperature dependent in the range 140–180 °C, while no improvement was found for higher temperatures, independently of the stirring speed. Likewise, temperature did not exert a marked effect on desorption kinetics.The irreversibility of the adsorption processes was confirmed from N2 and PNP adsorption analyses, although some differences were found. It is worth mentioning that increasing temperature did have a relevant influence on the adsorption regain of the adsorbents, being maximum for 200 °C (N2 and PNP adsorption efficiency values up to 61% and 80%, respectively). This might be attributed to the occurrence of some activating effect induced by water under these conditions. The study of the prevalence of desorption over adsorption for enhanced temperature was complemented by the development of an Arrhenius-based model.
No preview · Article · Jan 2014 · Journal of Supercritical Fluids The
[Show abstract][Hide abstract] ABSTRACT: In this work the thermal regeneration of activated carbons saturated with p-nitrophenol has been analysed. By thermogravimetry, it was possible to elucidate the different events taking place during the thermal treatment, and relate them to the type of adsorption in the interfacial system. It was found that the mass loss during thermal treatment comprises a complex process in which different stages are involved, such as drying, desorption of physisorbed adsorbate, breaking up of surface functional groups, cracking of products from adsorbate–surface-specific interactions, etc. The analysis of the textural and surface chemistry characteristics of the pristine and regenerated adsorbent confirmed the thermal desorption mechanisms. Moreover, a kinetic study based on temperature-programmed desorption and Suzuki models was performed, using thermogravimetry data at different heating rates (5–20 K min−1). From this analysis, the values of activation energy involved in each degradation step were estimated.
No preview · Article · Jan 2014 · Journal of Thermal Analysis and Calorimetry
[Show abstract][Hide abstract] ABSTRACT: Activated carbons previously used for p-Nitrophenol (PNP) adsorption were subjected to thermal reactivation in order to recover their initial porosity characteristics. Three activating agents were comparatively analysed (air, carbon dioxide and water steam). Regeneration results improved in the sequence air < CO2 < steam; steam activation almost removed all the adsorbate adsorbed on the carbon, achieving regeneration efficiency values up to 94% for N2 adsorption, and above 100% for PNP adsorption. The activation process did not cause a significant modification of the pore size distribution of the adsorbents, which remained microporous irrespective of the activating agent. The analysis of gases evolved was consistent with the chemical processes involved in the respective activations. There was a significant difference in the pattern followed by H2 in steam activations compared with CO2 and air. The prominence of water gas and water gas shift reactions were associated to this effect, which was also evident from the increase in CO and CO2 concentration.
No preview · Article · Dec 2013 · Fuel Processing Technology
[Show abstract][Hide abstract] ABSTRACT: In this work, the surface free energy of biomass-based activated
carbons, both fresh and impregnated with triethylenediamine, has been
evaluated. The contribution of Lifshitz van der Waals components was
determined by the model proposed by van Oss et al. The results obtained
allowed predicting the most probable configurations of the impregnant
onto the carbon surface and its influence on the subsequent adsorption
of radioactive methyl iodide.
No preview · Article · Oct 2013 · Applied Surface Science
[Show abstract][Hide abstract] ABSTRACT: This work studies the thermal regeneration of p-nitrophenol exhausted activated carbons by a novel method in which two reactors are connected in series in order to minimize the emission of the hazardous adsorbate and other heavy hydrocarbon molecules to the environment. For this task, previous one-stage runs were made, in which the ACs were thermally treated, studying the influence of heating rate and temperature on: (a) the final porosity properties of regenerated ACs, as well as the regeneration efficiency, and (b) the gas profiles (H2, CO and CO2) evolved during the process. It was found that these compounds were mainly released at high temperatures (above 450 °C), corresponding to the removal of the carbon surface functional groups and/or cracking of chemisorbed products.The use of a second reactor gave rise to a significant modification in the gas patterns, in comparison with one step experiments. H2, CO and CO2 where identified at higher quantities and their emission started at lower temperatures. These results allowed to conclude that the use of this additional gas treatment promotes the steam cracking of the physisorbed p-nitrophenol molecule, which is removed during the first stages of the regeneration process.
No preview · Article · Sep 2013 · Journal of Analytical and Applied Pyrolysis
[Show abstract][Hide abstract] ABSTRACT: The methyl iodide capture from a water vapour stream using 1,4-diazabicyclo [2.2.2] octane (DABCO) impregnated activated carbons is, for the first time, fundamentally described here at the atomic level by means of both molecular dynamics and grand canonical Monte Carlo simulations. A molecular dynamics annealing strategy was adopted to mimic the DABCO experimental impregnation procedure in a selected slit-like carbon pore. Predictions, restricted to the micropore region are made about the adsorption isotherms of methyl iodide, water and nitrogen on both impregnated and bare activated carbon models. Experimental and simulated nitrogen adsorption isotherms are compared for the validation of the impregnation strategy. Selectivity analyses of the preferential adsorption towards methyl iodide over water are also reported. These simulated adsorption isotherms sum up to previous experimental studies to provide an enhanced picture for this adsorption system of widespread use at nuclear plants HVAC facilities for the capture of radioactive iodine compounds.
[Show abstract][Hide abstract] ABSTRACT: Hydrothermal carbonization processes were studied under different conditions using two different biomass materials: walnut shell and sunflower stem. Coalification under mild conditions was promoted with the aim of increasing the calorific value of the solid hydrochar. Hydrocarbonization processes brought up an increase on the heating value of the materials up to 28.9 and 29.3 MJ kg− 1, for sunflower stem and walnut shell, respectively, which corresponds to an increase of 1.75 and 1.50 fold when compared with the natural biomass. Also, regarding the variables studied it was found that temperature and water/biomass ratio were more influent on the hydrocarbonization process than residence time. The hydrochars show negligible N2 adsorption at 77 K and interesting surface morphology, characterized by the formation of microspheres which grow as temperature increases. Moreover, hydrochars are acid in nature and present several oxygen functionalities, as observed from FT-IR and pzc analysis.
No preview · Article · Nov 2012 · Fuel Processing Technology